The invention relates to a safety switching arrangement for the switching on and switching off of the power supply of two actors which jointly set a working apparatus in operation in dependence on the switching on of the current. The actors are preferably relays which jointly control the working apparatus in the sense that the working apparatus is only switched on when both actors are active. If one of the actors is inactive then the working apparatus is switched off. The working apparatus can for example be a dangerous working machine which is only in operation on activation of both actors, whereas it is brought into its non-dangerous preferably switched off position when one or both of the actors have become inactive.
It is already known (DE-PS 28 31 089) to provide at least two contact incorporating switching outputs between a sensor which is for example formed as a light barrier and a machine control, with the switching outputs being formed by at least two relays which can be switched off independently from one another and which preferably have compulsorily guided contacts. In this way a dangerous state detected by the sensor is converted into a stop command which is transmitted by opening of the switching outputs for the subsequent machine control. The machine control thereupon converts the controlled system from the endangering operating state into a switched off safe operating state in which the two switching outputs are open.
In the known safety switching arrangement, the safety is achieved by redundant execution of the switching path and in this way it is ensured that during the endangering state (switching on of the working apparatus) at most one switching off path looses the switching off function as a consequence of the fault which has arisen. Before each initiation of the endangering, switched-on, state of the machine of the working apparatus, the two switch-off paths can be checked to see that each indicates a safe operating state. If a fault is uncovered, for example a welded switching output or a short circuit in the connection lines from the switching output of the machine control, a latching circuit keeps the second fault-free switch-off path in a safety switching state independently of the sensor signal.
The known safety switching arrangement has the disadvantage that faults in the output circuit which arise during the continuous operation in each endangering state (switched on working apparatus) are not detected. Through an accumulation of faults the danger exists that the machine will not be converted into a safe operating state on the appearance of a stop signal from a sensor, i.e. will not be switched off. In particular it is disadvantageous that the examination of the switching-off paths does not take place automatically in a defined time interval, but rather must take place through administrative instructions to an operator.
Short circuits between connection lines to the machine control cannot be uncovered by the known solution. This fault situation can only be partly controlled by restrictive rules of use, which however stand partly in contradiction to technical rules.
Switches with contacts are subject to wear as consequence of which regular repair costs and machine downtimes arise.
Finally, mechanical contacts must be protected against burning away by arc formation on switching off inductive loads in order to ensure useful working lives for the contacts. In this way further costs and possible sources of faults arise.